1. Field Of The Invention
The invention relates generally to the field of ultrasonic examination, particularly to medical diagnostic ultrasonic examination. The invention relates more specifically to a system for facilitating the evaluation of performance characteristics of ultrasonic examination systems by electronic acoustic synthesis of static and dynamic structures within the ultrasonic system's field of view, rather than by placing a phantom tissue mimicking object in the field.
2. Background Art
Apparatus and systems are known for employing mechanical ultrasonic energy for non-invasive examination of internal body structure or condition of a patient or other subject. All of such system employ a transducer having at least one transducer element, along with transmitter circuitry, receiver circuitry, signal processing means and some type of display or other device for recording the results of the examination.
The transducer element or elements are often piezoelectric crystals which convert mechanical sound energy to electrical energy, and vice versa. In an examination, the transducer is placed upon the surface of the patient's body near the area sought to be studied. The transmitter is coupled to the transducer, and produces electrical signals which cause the transducer to propagate mechanical ultrasonic signals into the patient's body, either continuously or in pulses. The transmission period is relatively short in duration. Following the transmission period, the transducer is coupled to the receiver circuitry for a "listening" period many times longer than the transmission period.
Incident acoustic energy travels through body tissue at a high rate of speed.
Different organs and structures of the body have different acoustical impedances. Where a change in acoustical impedance occurs, an acoustical interface is defined. Incident ultrasonic radiation impinging upon an acoustical interface produces acoustic echoes which are reflected generally back toward the direction from which the incident ultrasonic energy originated.
The "listening" period is defined with sufficient duration to encompass the time at which an acoustic echo occurring in the region of interest will return to the transducer. Echoes striking the transducer cause the transducer to produce electrical signals which are representative of characteristics of those ultrasonic echoes. The electrical signals are transmitted from the transducer to the receiver, which often performs a buffering and/or amplification function. The electrical signals output from the receiver are transmitted to processing circuitry which sometimes includes means for digitizing the signals prior to processing. Storage means is often provided for preserving the signals. Some display device is included to present an image, histogram, or other tangible representation of the signals and of the location and intensity of the echoes that caused them.
The acoustic echoes, and hence the stored signals representing them, depend upon the physical properties of the acoustic interface or boundary which caused the generation of the echo signal. Such physical characteristics include the size of the boundary, the difference in acoustic impedance between the two media which define the boundary, and whether the interface happens to be in motion.
The stored electrical signals thus can be used to construct a visual image of the boundary, or to define in other ways one or more of the location or characteristics of the acoustic interface. This technique is generally referred to as ultrasonic imaging.
Other forms of ultrasonic examination exist. For example, the Doppler effect can be employed to detect and quantify motion of an acoustic interface or boundary. In ultrasonic systems for utilizing Doppler, a transducer propagates ultrasonic energy into the subject at a predetermined frequency. If an acoustic interface is in motion away from the ultrasonic source, the received ultrasonic echo frequency will be slightly lower than the transmitted frequency. If the ultrasonic interface is in motion toward the ultrasonic source, the received frequency will be slightly higher than that which was transmitted.
Such Doppler systems are often used to measure blood flow. The ultrasonic source directs incident ultrasonic energy along a blood vessel. Since blood is not a homogeneous fluid, but rather contains corpuscles which have acoustic impedance different from that of the surrounding fluids, ultrasound will be reflected from the blood corpuscles. If the corpuscles in the blood are moving away from the source, the return frequency will be lower than the incident frequency. If the blood if flowing toward the source, the return frequencies will be somewhat higher. The amount of difference between the transmitted and received frequencies indicates the velocity with which the acoustic interface is moving.
Another type of ultrasonic examination is known as color flow mapping.
Color flow mapping ultrasonic systems are a more sophisticated version of simple Doppler systems. Color flow mapping employs the Doppler effect to produce a multi-color display for simultaneously indicating various velocities of moving fluid which are distributed across the system's field of view. Color flow mapping systems substantially simultaneously observe and record the distribution of observed velocities across the field of view, and produce a display which indicates those different velocities in a predetermined color scheme.
It is necessary, from time to time, to evaluate and/or calibrate the performance of ultrasonic examination systems. It has been proposed to do this by placing a mechanical object, often called a "phantom", in the field of view of the ultrasonic examination system. The phantom is made of or contains a material which has an ultrasonic impedance resembling or mimicking that of body tissue of a particular organ or body structure.
To evaluate or calibrate performance, the examination system is operated with the phantom in the field of view. The image or other tangible representation of the phantom which is produced by the examination system is evaluated against an ideal image or other representation of the phantom which would be expected if the system were operating perfectly.
Other devices, or phantoms, used for testing performance of an ultrasonic Doppler system do so by mechanically moving an echogenic fluid within the field of view of the ultrasonic system. The moving field impresses a Doppler shift in frequency upon the ultrasonic signal transmitted from the ultrasonic examination system. This shift infrequency is then detected in the echo returned to the examination system.
Such Doppler phantoms, however, are very bulky. They require fluid pumps, motors and piping to create the fluid flow. In using them, it is very difficult to control the flow velocity of the fluid and they can only generate one Doppler shifted signal at a time. Furthermore, the flow that they do create is not a pure quantity that can be used for calibration purposes. This is due to the physics of the fluid flow in the rigid tubes of the phantom. Blood flow in an actual blood vessel exhibits somewhat different characteristics than fluid flow in a rigid tube, since blood vessels are somewhat elastic in nature.
For even in the case of simple laminar flow, the distribution of velocities across the rigid tubes is non-linear. The fluid in the center of the tube moves at a higher velocity than the fluid adjacent the tube walls. This makes it very difficult, if not impossible, to use the prior art Doppler phantom as an accurate means for calibrating Doppler systems.
Furthermore, to be able to generate multiple, simultaneous, calibrated Doppler shifts representing a plurality of velocities needed to accurately test color-flow imaging systems would be impossible to achieve with the prior art Doppler phantoms.
Examples of known types of ultrasound phantoms are described in the following United States Letter Patent, each of which is hereby expressly incorporated by reference: U.S. Pat. No. 4,843,866, issued on Jul. 4, 1989, to Madsen et al. for ULTRASOUND PHANTOM; U.S. Pat. No. 4,974,461, issued on Dec. 4, 1990 to Smith et al. for ANTHROPOMORPHIC CARDIAC ULTRASOUND PHANTOM.
It is an object of the present invention to provide apparatus and circuitry for evaluating the performance of an ultrasonic examination system which is compact, lightweight, and which does not require the placement of a mechanical phantom in the field of view of the ultrasonic system, and which is capable of the simultaneous generation of multiple simulations of both moving and stationary ultrasonic interfaces within the system's field of view.